The present invention relates generally to a 1:1 unbalanced to balanced transmission line transformer (commonly known as a balun).
High frequency electric signals can be transmitted in two often occurring ways, namely in balanced and unbalanced transmissions. Unbalanced transmission lines use coaxial cable and signals are transmitted via transmission line mode. Current flows in one direction on the center conductor and in the opposite direction on the inside surface of the outer conductor (shield). The outer conductor is normally connected to ground.
Balanced and unbalanced transmissions are often mixed in various applications, such as in radio systems. It is therefore necessary to enable a balanced signal to be converted to an unbalanced signal, and vice versa, with the smallest losses possible. Baluns are used to this end.
The 1:1 Balun transformer has been known for some time and much information can be found on the subject. For example, Jerry Sevick has covered the subject extensively in his book titled xe2x80x9cTransmission Line Transformersxe2x80x9d, 2d Edition, 1990; Published by the American Radio Relay League.
FIG. 1 shows a prior art balun 10, employing a multiple turn coaxial transmission line winding 12, with a center conductor 14 and an outer conductor 16 wound on a toroidal ferrite core 18. Terminals 20 and 22 are the balanced end terminals, while 20a and 22a form the unbalanced terminals of balun 10. Terminal 22a is connected to common ground.
Thus, a typical transmission line balun consists of several turns of either bifilar or coaxial transmission line wound on a high permeability magnetic core such as ferrite. The high longitudinal impedance between the ends of the windings isolate the balanced from the unbalanced end of the balun, choking off the flow of non-transmission line current. The useful low frequency response of the balun is limited to that frequency where inadequate choking inductance allows the flow of conventional current on the windings.
The upper frequency response is limited by (i) the match between transmission line and load impedance, (ii) undesired parasitic effects and (iii) resonances in the pass band which limit the electrical length of the winding. The upper frequency requirement for a practical balun will dictate an electrical length in the order of about 0.1 wavelength, limiting the number of turns on the core. The useful low frequency is then limited by the choice of core material, and the choking inductance produced by the limited number of turns. High power baluns of a kilowatt or more have a typical useful bandwidth of about 20:1.
What is desired, therefore, is a very high power, ultra wideband balun featuring parasitic reduction and techniques to extend both the high and low frequency ends of the useful range.
Accordingly, it is an object of the present invention to provide an ultra wideband high power transmission line balun transformer.
Another object of the present invention is to provide a balun transformer having the above characteristics and which may be scaled to accommodate very high power levels.
A further object of the present invention is to provide a balun transformer having the above characteristics and which exhibits low loss and low voltage standing wave ratio (VSWR) characteristics.
Still another object of the present invention is to provide a balun transformer having the above characteristics and which employs a method of reducing frequency limiting parasitic effects.
Yet a further object of the present invention is to provide a balun transformer having the above characteristics and which incorporates resonance suppression techniques.
Still a further object of the present invention is to provide a balun transformer having the above characteristics and which has an electrical length that is not limited to a fraction of a wavelength.
Yet still another object of the present invention is to provide a balun transformer having the above characteristics and which employs a method of increasing the low frequency longitudinal impedance thereof.
Yet still a further object of the present invention is to provide a balun transformer having the above characteristics and which employs a method of reducing the resistive loss component of the longitudinal impedance therein.
These and other objects of the present invention are achieved by provision of a balun transformer which includes a coaxial transmission line terminating at one end with balanced terminals and terminating at another end with unbalanced terminals. At least two impedance segments are connected in series between the balanced terminals and the unbalanced terminals, each of the at least two impedance segments being formed by a magnetic core and at least one winding of the coaxial transmission line wound about the magnetic core.
Preferably, each of the magnetic cores is toroidal in shape. It is also preferable that each of the magnetic cores is formed from a ferrite material.
Preferably, the at least two impedance segments comprises a first impedance segment connected to the balanced terminals and a second impedance segment connected between the first impedance segment and the unbalanced terminals, and wherein a number of windings of the coaxial transmission line wound about the core of the second impedance segment is greater than a number of windings of the coaxial transmission line wound about the core of the first impedance segment. In this case, it is also preferable that the core of the second impedance segment has a permeability greater than or equal to a permeability of the core of the first impedance segment and exhibits a loss characteristic greater than or equal to a loss characteristic of the core of the first impedance segment.
Most preferably, a third impedance segment is connected between the second impedance segment and the unbalanced terminals, and wherein a number of windings of the coaxial transmission line wound about the core of the third impedance segment is greater than or equal to a number of windings of the coaxial transmission line wound about the core of the second impedance segment. In this case it is also preferable that the core of the third impedance segment has a permeability approximately equal to a permeability of the core of the second impedance segment and exhibits a loss characteristic approximately equal to a loss characteristic of the core of the second impedance segment.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.